Field of the Invention
Injection-moulding apparatuses of that kind are already known and are described hereinafter in relation to a packaging machine in which paper which is coated on both sides with plastics material is drawn in the form of a web from a roll, cut up into individual portions and shaped by means of a longitudinal sealing seam to form a paper tube which is of round cross-section and which is pushed on to a mandrel, which is also of round cross-section, to act as an injection-moulding core, and which terminates with its edge in the injection-moulding position in the mould cavity of the tool (injection-moulding core and outer mould) in such a way that the heated liquid plastics material flows into the mould cavity and embraces the round edge of the paper tube in such a way as to close it off completely. In that way, in the known machine, the tubular paper casing is closed at one end by the cover for the cover comprises plastics materal without a carrier or backing material and for example also provides the pouring-out opening at the same time. The cover is anchored to the end edge of the tubular paper casing, is supported against same, and at the same time also seals off the open cut face of the end edge of the tubular paper casing. For, if a web of paper which is coated with plastics material on both sides is cut, then the cutting operation gives rise, along the cut edge which after the tubular casing has been formed becomes the circular end edge thereof, to an unprotected edge which is without any plastics coating and which must be protected from the ingress of liquid because otherwise the liquid penetrates into the paper fibres between the layers of plastics and destroys the pack from that cut edge. Therefore the known injection-moulding machine has such a mould cavity that the upper round end edge of the tubular paper casing does not terminate at the edge of the mould cavity but in the interior thereof, with the result that the plastics material which subsequently completely fills the mould cavity is not only applied to the free end edge which was the cut face, but is also applied on both sides beside same and thus closes off the upper edge of the paper tube to make it liquid-tight.
The packaging machine for producing liquid packs which are at least partly round in cross-section operated satisfactorily in that fashion. However, operators have now gone over to producing liquid packs of quadrangular cross-section using the same injection-moulding procedure. The difference in relation to the round pack, in terms of the tool used, is only that the mandrel is also of quadrangular cross-section, by virtue of forming four outer planes, and the tubular paper casing of a corresponding configuration is pushed on to the square mandrel, in a comparable position. It was thought otherwise that no changes in the injection-moulding procedure are involved.
Now however it has been found that the flat side wall of the quadrangular liquid pack has leakage points at the point of attachment of the cover. Initially, because of the tool portions being carefully manufactured, in particular in regard to the end regions of the injection-moulding core and also the outer mould, it was not possible to find the reasons for the flaws in regard to the leaking points, until, in the region of the flat edge of the paper tube, at a spacing from the four corners, some injection-moulded bead portions were detected on the outside, and portions of reduced thickness were found, on the inside of the liquid pack. Investigation then showed that one or other of the flat side walls of the liquid pack or the tubular paper casing had not been applied precisely against the flat wall of the outer mould, in the region of the upper edge of the tubular paper casing.
The invention was therefore based on the object of improving the injection-moulding apparatus of the kind set forth in the opening part of this specification, in such a way that, when dealing with a paper tube of quadrangular cross-section, the flat walls of the tube do not suffer from flexing and deflection and the injection moulding is of a reliable nature throughout.
While, in the case of the liquid pack of round cross-section, with the round tubular paper casing, there were adequate support forces by virtue of the use of the paper material and the round configuration thereof, so that the upper edge of the tubular paper casing was satisfactorily applied to the wall of the outer mould, which was in the form of a cylindrical surface, and the heated plastics material flowing into the mould was also unable to press that upper edge away from the wall of the mould, the investigation in accordance with the invention showed that such stiffening and holding forces were lost in the upper edge of the paper tube by virtue of its flat configuration between the corners thereof. In actual fact it was then found that, in the injection-moulding operation, the flat walls in the region of the upper edge flex and move into the mould cavity in any random fashion, wherever there is initially a free space which is first filled by the plastics material flowing into the mould. If for example the middle of a flat upper edge of the tubular paper casing is considered, it is possible to envisage that flat upper edge flexing away from the wall of the outer mould inwardly towards the injection-moulding core, with the result that plastics material flows into the space between the wall of the outer mould and the outside surface of the flat upper edge of the paper tube, and forms the above-described plastics bead which was initially found, at a location at which it is not only superfluous but is even undesirable for the reason that the amount of plastics material which is superfluous at that point is missing from the oppositely disposed inward side and therefore results in leakage points or at least a weakening of the anchoring forces by means of which the cover is subsequently to be secured to the paper casing. The smallest amount of load on the filled liquid pack then results in that weakened point breaking open.